As magnetic recording media for use in audio, video, computer (disc, memory tape), etc. there has been normally used a magnetic recording medium comprising on a non-magnetic support a magnetic layer having a ferromagnetic powder dispersed in a binder.
Heretofore, such a magnetic recording medium has comprised a magnetic layer containing a ferromagnetic powder, a binder and an abrasive material having a Mohs hardness of 8 or more. Among these components, the abrasive material has served to suppress stain on VTR heads to provide invariably excellent recording of visual and audio signals and other data. To this end, various abrasive materials have been proposed, and some are disclosed in JP-B-49-39402 (the term "JP-B" as used herein means an "examined Japanese patent publication"), JP-A-57-183628 and JP-A-57-179945 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"), and U.S. Pat. No. 3,630,910.
In general, the magnetic layer in the magnetic recording medium thus prepared has powder components (e.g., ferromagnetic powder, non-magnetic grains such as abrasive material) rigidly fixed to the film and thus can be considered fairly smooth. However, the inventors' study has shown that the magnetic layer has minute protrusions and insufficiently fixed powder components present in its surface. This is attributed to the fact that the ferromagnetic powder and non-magnetic grains having a hardness as high as 8 or more as calculated in terms of Mohs hardness such as alumina and chromium oxide grains can hardly be dispersed or tend to agglomerate with time even if dispersed. If such an agglomerative magnetic coating is coated on a non-magnetic support, a magnetic layer having minute protrusions and insufficiently fixed powder components having a high Mohs hardness existing in its surface layer is formed. When incorporated in a video tape, for example, such a magnetic layer causes the magnetic head to wear in a short period of time during running. Such minute protrusions and insufficiently fixed powder components may leave the magnetic layer and adhere to the magnetic head, causing clogging of the magnetic head or occur of dropouts. Further, the minute protrusions and insufficiently fixed powder components which have left the magnetic layer during running may cause ferromagnetic powder to leave the surface of the magnetic layer, gradually reducing the content of ferromagnetic powder in the vicinity of the surface of the magnetic layer. This causes another problem that as running is repeated, the electromagnetic conversion characteristics (output) are lowered.
In recent years, these magnetic recording media have been required to provide high density recording. For this purpose, an increase in output and a reduction in noise by supersmoothening of the surface of the magnetic recording medium, finer division of ferromagnetic powder, fine division of metal, high packing, and reduction in the thickness of magnetic recording medium have been demanded. Further, in order to improve the recording density and image quality, it is necessary to reduce the speed of writing into and reading from the magnetic recording medium in VTR, to modify the recording system (i.e., analog system to digital system), or to reduce the recording width (5 to 20 nm) or the minimum recording wavelength (0.1 to 0.9 .mu.m). In the helical scan VTR, the tendency has been that the head cylinder speed is 5,400 rpm or more and the speed of tape relative to the magnetic head is far more than 20 m/sec. In order to provide a magnetic recording medium with such a high output or excellent adaptability to high speed sliding movement, it is indispensable to secure sufficient running stability for the magnetic recording medium in the VTR running system or head and cylinder systems. To this end, carbon black or various lubricants based on organic compounds are used besides the foregoing abrasive materials.
These magnetic recording media have a great disadvantage of insufficient affinity for VTR heads. If the magnetic recording medium is thinned and super-smoothened for high density recording, it is extremely difficult to reduce its coefficient of friction with a VTR head. When the coefficient of friction rises, the surface of the magnetic layer is scraped, contaminating the head. In order to avoid such trouble, an abrasive material as mentioned above is incorporated in the magnetic layer. However, if such an abrasive material is incorporated in such an amount that a sufficient effect of eliminating stain on the magnetic head can be exerted, the head wears to a remarkably larger extent. In order to cope with this problem, it is proposed that the magnetic head be made of a harder material. Accordingly, the amount of an abrasive material to be incorporated in the magnetic layer must be again increased.
In other words, an effective method for coping with the rise in the coefficient of friction of the magnetic recording medium with the VTR head in the prior art is to incorporate an abrasive material and a lubricant in the magnetic recording medium. The stain on the magnetic head can be effectively eliminated by incorporating an abrasive material in the magnetic layer. If such an abrasive material is incorporated in the magnetic layer in a sufficient amount, the wearing of the VTR by the magnetic recording medium is remarkably increased. In order to cope with this problem, it is proposed that the magnetic head be made of a harder material. Accordingly, the amount of an abrasive material to be incorporated in the magnetic layer must be again increased. Thus, either head wearing or head stain must be sacrificed in the prior art. In order to lower the coefficient of friction of the magnetic recording medium with VTR head against the reduction in output caused by the thinning and supersmoothening of the magnetic recording medium for better affinity for VTR head, i.e., high density recording, a lubricant is incorporated in the magnetic layer besides the foregoing abrasive material. The addition of such a lubricant for the reduction of the coefficient of friction plasticizes the magnetic layer, causing output reduction or head stain. Thus, the reduction in the coefficient of friction and the output drop are very difficult to balance.
It has also been proposed that the content of an abrasive material in the surface of the magnetic layer be defined to improve the durability of the magnetic layer. For example, U.S. Pat. No. 4,670,337 (corresponding to JP-A-61-57036) discloses a magnetic recording medium which comprises abrasive grains having a Mohs hardness of 6 or more in a proportion of 3 to 15% by weight based on the weight of magnetic powder wherein the average number of grains per unit area of the surface of the magnetic layer is defined to be 0.25/.mu.m.sup.2 or more. Further, JP-A-60-93631 discloses a magnetic recording medium comprising non-magnetic abrasive grains having a Mohs hardness of 7 or more in a proportion of 20 pieces or more per 100 .mu.m.sup.2 of the surface of the magnetic layer. However, none of these proposals can satisfy all the requirements in initial head abrasion, output difference by VTR and low humidity output.
In the field of magnetic discs, it is known to effect polishing for surface smoothening. For example, a magnetic disc substrate surface smoothening apparatus which continuously supplies an abrasive tape onto the surface of a head slider (as disclosed in JP-B-58-46768) and a magnetic disc substrate surface smoothening method which comprises moving back and forth a polishing head having on a slider surface a rough surface portion made of an abrasive grain layer containing hard grains in contact with a magnetic disc substrate while the magnetic disc substrate is rotated (as disclosed in JP-B-58-46767) are known.
As an approach for preventing the foregoing clogging and the occur of outputs to reduce the output drop after repeated running there has been proposed a process for the preparation of a magnetic recording medium which comprises abrading insufficiently fixed powder components on the surface of the magnetic layer as disclosed in JP-A-62-172532 and JP-A-63-98834. That is, the surface of a magnetic layer which has been calendered is polished with a grinding means having a high hardness such as a diamond wheel, a fixed blade or a rotary blade to remove minute protrusions and insufficiently fixed powder components or attachments and hence inhibit the output drop caused by clogging, dropout and prolonged running.
JP-A-63-183619 discloses a process for the preparation of a magnetic recording medium which comprises coating a magnetic layer, drying the material, calendering the material to cure the material, pushing a blade having an edge made of sapphire or diamond against the surface of the magnetic layer while the latter is running, and then polishing the surface of the magnetic layer with a cylindrical diamond grindstone having a grain diameter of 0.5 to 4 .mu.m. Further, JP-A-3-162717 discloses a process for the preparation of a magnetic recording medium which comprises coating a magnetic layer, calendering the material, and then polishing the surface of the resulting magnetic layer with an ultra-hard blade. However, none of these proposals can satisfy all the requirements in initial head abrasion, output difference by VTR and low humidity output.